Firstly, lets define a beam level. It is the space between the top surface of a beam to the underside of the beam above it. The clearances are originally set out in the erection of the racking structure. The details on the beam levels are listed on the Safe Working Load Limit (SWL) Sign on the end of each of the racking aisles. The installation of these by your racking provider was at the same time as the racking structure installation. Your provider will have copies on hand of the SWL Signs that you need to display in your warehouse.